926 THE ORGANS OF SENSE. 



rows of epithelial cells, more elongated than those found on the internal side of 

 the inner rod, but, like them, furnished with minute hairs or cilia. These are 

 termed the outer hair-cells, in contradistinction to the inner set, which are termed 

 the inner hair-cells. They are attached by their bases to the basilar membrane, 

 whilst from the opposite extremity a brush of hairs or cilia projects through the 

 reticular membrane. They are continuous externally with the cubical cells on 

 the lateral part of the basilar membrane. 



The reticular lamina or membrane of Kolliker is a delicate framework perfo- 

 rated by rounded holes. It extends from the inner rods of Corti to the external 

 row of the outer hair-cells, and is formed by several rows of " minute fiddle- 

 shaped cuticular structures," called phalanges, between which are holes for the 

 projection of the cilise of the outer hair-cells. 



Covering over these structures, but not touching them, is the membrana tec- 

 toria, or membrane of Corti, which is attached to the vestibular surface of the 

 lamina spiralis close to the attachment of the membrane of Reissner ; it courses 

 over the denticulate lamina, and, passing outward parallel to the basilar mem- 

 brane, is blended with the ligamentum spirale on the outer wall of the spiral 

 canal. 1 



The inner surface of the osseous labyrinth is lined by an exceedingly thin 

 fibro-serous membrane, analogous to a periosteum from its close adhesion to the 

 inner surfaces of these cavities, and performing the office of a serous membrane 

 by its free surface. It lines the vestibule, and from this cavity is continued into 

 the semicircular canals and the scala vestibuli of the cochlea, and through the 

 helicotrema into the scala tympani. This membrane is continued across the 

 fenestrae ovalis and rotunda, and consequently has no communication with the 

 lining membrane of the tympanum. Its attached surface is rough and fibrous, 

 and closely adherent to the bone ; its free surface is smooth and pale, covered 

 with a layer of epithelium, and secretes a thin, limpid fluid, the aqua labyrinthi, 

 liquor Cotunnii, or perilymph (Blainville). 



The Membranous Labyrinth. 



The membranous labyrinth (Fig. 551) is a closed sac, containing fluid, on the 

 walls of which the ramifications of the auditory nerve are distributed. It has the 

 same general form as the cochlea, vestibule, and semicircular canals in which it 

 is enclosed, but is considerably smaller, and the vestibular and canalicular por- 

 tions are more or less surrounded by the perilymph. 



The scala media, already described in connection with the cochlea, is closed 

 above and below. The upper blind extremity is attached to the cupola at the 

 upper part of the helicotrema ; the lower end fits into the angle at the com- 

 mencement of the osseous lamina on the floor of the vestibule. Near this blind 

 extremity the scala media receives the canalis reuniens (Fig. 551), a very delicate 

 canal by which the ductus cochlearis is brought into continuity with the saccule. 



The vestibular portion consists of two sacs, the utricle and the saccule. 



The utricle is the larger of the two, of an oblong form, compressed laterally, 

 and occupies the upper and back part of the vestibule, lying in contact with the 

 fovea semi-elliptica. Numerous filaments of the auditory nerve are distributed 

 on the wall of this sac, and its cavity communicates behind with the membra- 

 nous semicircular canals by five orifices. It also sends off a minute canal into 

 the aqueductus vestibuli, which unites with the ductus endolymphaticus, a similar 

 but somewhat larger tubular prolongation from the saccule. 



The saccule is the smaller of the two vestibular sacs ; it is globular in form, 

 lies in the fovea hemispherica near the opening of the vestibular scala of the 

 cochlea, and receives numerous nervous filaments which enter from the bottom 

 of the depression in Avhich it is contained. Its cavity communicates with that 

 of the scala media by means of the canalis reuniens and with that of the utricle 

 in the manner just mentioned. 



1 In Fig. 550 only the inner half of the membrane is represented. 



